1. Field of the Invention
The present invention relates generally to a flyback converter, and more particularly to a flyback converter with primary side and secondary side feedback control.
2. Description of the Prior Art
The power supply used for electronic products generally requires electric isolation between an input end and an output end thereof. The electric isolation can be realized, for example, by a transformer, and one of the popular schemes is a flyback converter scheme. In the flyback converter scheme, the output power can be adjusted by controlling pulses applied to the primary winding of the converter. More particularly, the turn-on time of the pulses increases when the output power is to be increased, and the turn-on time of the pulses decreases when the output power is to be decreased.
FIG. 1 shows a schematic diagram of a prior art flyback converter with secondary feedback control. The prior art flyback converter shown in FIG. 1 comprises a full bridge rectifier 12A, a PWM controller 14A, a switch unit 16A, a transformer 20A, an output filter 32A and a feedback unit 30A. The full bridge rectifier 12A receives an AC input such as a wall socket power to generate a rectified power when the flyback converter is used in an AC-DC conversion application. The rectified power is connected to ground through the primary winding Wp and the switch unit 16A. Energy is coupled to the secondary winding Ws and the auxiliary winding Waux when the rectified power is delivered to the primary winding Wp. The secondary winding Ws send the coupled energy to a load (not shown) through the output filter 32A.
The feedback unit 30A senses an output power of the load (for example the feedback unit 30A senses the current or voltage of the load) to generate a feedback signal, and the feedback unit 30A sends the feedback signal to the PWM switch 14A. The PWM switch 14A controls the switch unit 16A based on the feedback signal or a control signal from an external control unit (not shown). In one popular implementation, the output voltage Vout is divided into a voltage Vdiv through a resistor network. The voltage Vdiv controls the shunt regulator TL431, which operates as an error amplifier and generates a current proportional to the difference of the voltage Vdiv and an internal regulated voltage in the shunt regulator TL431. The current generated will be converted into a feedback voltage and sent to the primary side through the optocoupler OPTO, thus stabilizing the flyback converter.
One effort for improvement of electronic products is to minimize power consumption in the standby operation in order to prolong battery operation time as environmental protection becomes a serious issue. For the above-mentioned flyback converter with secondary feedback control, an error amplifier and optocoupler are employed to send a feedback signal to the primary side and the output power needs continuous monitoring in standby mode. This consumes considerable electric power in the standby operation of the flyback converter with secondary feedback control.
FIG. 2 shows a schematic diagram of a prior art flyback converter with primary feedback. The flyback converter with primary feedback senses electric power of the auxiliary winding Waux by the PWM controller 14B to provide primary side feedback instead of monitoring secondary side power with the optocoupler. However, the flyback converter with primary feedback cannot precisely sense the output power from the load due to the nonlinear property of the transformer 20A and the feedback unit 30A. Moreover, the flyback converter with primary feedback control has the drawbacks of poor output regulation and inapplicability to high power (for example above 15 W) application. Besides, primary-side control does not work well in Continuous Current Mode (CCM). Many PWM converters operate at CCM mode when in heavy load condition hence primary-side control has its limitation.
As can be seen from above description, the prior art flyback converter lacks the ability to provide both primary side and secondary side feedback control, and cannot meet the requirements of power saving and precise control simultaneously.